Expose fields in FieldPhraseList and WeightedPhraseInfo with public getters [LUCENE-3512]

Currently, to write a custom `FragListBuilder` implementation or custom highlighter, it probably will have to be placed in the `org.apache.lucene.search.vectorhighlight` package so that it has access to the fields in `FieldPhraseList` and its inner class `WeightedPhraseInfo`. It seems that in recent releases, getters were added for fields in many other classes in `vectorhighlight`, and I think it makes sense to add them for fields in `FieldPhraseList` as well to support such custom implementations.

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Migrated from [LUCENE-3512](https://issues.apache.org/jira/browse/LUCENE-3512) by Kelsey Francis
Attachments: [LUCENE-3512.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3512/LUCENE-3512.patch)

Locate FieldPhraseList and its inner WeightedPhraseInfo in org.apache.lucene.search.vectorhighlight, then compare nearby vectorhighlight classes that already expose public getters. Add public getters for the needed fields so custom FragListBuilder and highlighter implementations can use them without package access; done means the fields are externally readable.

Written by the indexing model from the issue text.